Magnitude types

Magnitudes can be based on any of the following:
Mw - estimates of the seismic moment
Mb - maximum amplitudes of teleseismic body waves
Ms - surface waves
Ml - local recordings
Md - signal duration

More Information
The USGS website has a FAQs section that answers some common questions about earthquakes and how they are measured: http://earthquake.usgs.gov/learning/faq.php

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